A semiconductor laser has two distinct regions of operation. At low currents, the ratio .eta..sub.1 of the device's optical output L to input current I curve is small and the laser has a small bandwidth. As input current I is increased, it eventually reaches "threshold" current (I.sub.th) and the device enters the lasing mode where the ration .eta..sub.2 is much greater than .eta..sub.1. Above threshold, the laser can have a very large bandwidth. In a digital application, the two most important operating points on the L-I curve are the logic ZERO and logic ONE optical output levels, L.sub.0 and L.sub.1. For optical communications, it is desirable to control the peak output level, L.sub.1, and the "extinction ratio" (.epsilon.=L.sub.1 / L.sub.0) and to maximize the switching bandwidth of the laser.
In high speed applications, important factors such as bandwidth, turn-on delay and spectral purity are dependent on the proximity of the logic ZERO operating point, I.sub.0, to the threshold current (I.sub.th) level. In these high frequency applications, where precise location of the logic ZERO operating point is critical, and in low cost applications, where there is no laser cooling, it is extremely important to have some form of laser stabilization to allow exact control of both laser bias and modulation current parameters across a range of device characteristics, temperature and time.
Prior art laser control circuitries are arranged to set the laser bias current near I.sub.th such as the circuits described in U.S. Pat. No. 4,680,810 issued to R. G. Swartz on Jul. 14, 1987, or to maintain average laser output power at some predetermined level, such as described in U.S. Pat. No. 4,009,385, issued to D. D. Sell on Feb. 22, 1977.
In the Sell patent, in particular, a single loop feedback control maintains the average optical power. The biasing of the laser is derived in response to the difference between a signal derived from a reference level and a signal derived from the laser light output.
Another known biasing scheme uses threshold tracking, where a monitoring circuit is used to fix the dc bias current at I.sub.th. For example, the previously-identified Swartz patent discloses a stabilization technique which uses the second or third derivatives of the voltage across a laser with respect to the current through the laser, i.e., d.sup.2 V/dI.sup.2 or d.sup.3 V/dI.sup.3, to indirectly control L.sub.0. The d.sup.2 V/dI.sup.2 curve exhibits two minima, one at threshold current I.sub.th and one at low bias currents. While the use of d.sup.2 V/dI.sup.2 results in improved threshold current (I.sub.th) tracking, the existence of two minima makes it more difficult to use a feedback circuit to establish a bias current I.sub.0 below the threshold current I.sub.th.
Reliable laser biasing is a continuing problem, especially in applications involving high performance and in low cost applications where uncontrolled laser temperature leads to wide variations in laser parameters. Moreover, in certain applications changes in the slope .eta..sub.2 as well as I.sub.th with temperature and time should be controlled. Prior art arrangements have not accounted for both parameter changes.